This invention relates to an automotive vehicle braking unit with a vacuum brake power booster arranged between the brake pedal and the master brake cylinder. The braking unit is furnished with at least two power chambers divided from each other by a movable wall. The first one of the power chambers is connectible to a vacuum source and the second chamber is ventable through a control valve actuatable by means of a piston rod coupled with the brake pedal. To generate a boosting power which is proportional to the brake pedal effort, wheel brake cylinders are connected through brake lines to a primary and a secondary pressure chamber of the master brake cylinder. Sensors are associated with the wheels to be braked to detect the rotating behavior of the wheels in order to determine any locking and the output signals of which are transmittable to a central electronic control system. Control signals of the control system control hydraulic fluid inlet and outlet valves which are electromagnetically actuatable and are inserted in the brake lines for the purpose of slip control.
Such a braking unit is known for example from the published German patent application No. 36 27 000. The special feature of this known braking unit operating with an auxiliary hydraulic pressure supply system is that in order to quickly reduce the brake pressure generated by the master brake cylinder in the event of a slip control action. The pistons of the master brake cylinder are furnished with central control valves which open hydraulic fluid connections between a hydraulic fluid reservoir and the pressure chambers of the master brake cylinder in the released position of the brake and which close the hydraulic fluid connections in the applied position of the brake. The brake lines are in connection, through intake lines into which non-return valves are incorporated, with the motor-driven pumps of the auxiliary pressure supply system whose suction connections are linked to the hydraulic fluid reservoir through a suction line.
In the known braking unit, a considerable amount of design effort is required to generate and to control the auxiliary hydraulic pressure, to retain the pistons of the master brake cylinder and to safeguard the braking function in the event of a failure of individual brake circuits.
It is, therefore, an object of the present invention to provide a braking unit in such a manner that an increased reliability in operation is achieved simultaneously with a major reduction of the manufacturing and assembly costs involved. In addition, the pistons of the master cylinder and the brake pedal is to be restored completely after each slip control action. Finally, the braking unit should form a simple anti-locking system.